In a magnetic recording medium used in a conventional video tape recorder (to be referred to as a "VTR" hereinafter) or the like, a magnetic layer is formed on one major surface (to be referred to as a "front surface" hereinafter) of a film-like non-magnetic support member, a protective layer is formed on the magnetic layer to protect it, and a back-coat layer for improving the characteristics of the magnetic recording medium is formed on the other major surface (to be referred to as a "rear surface" hereinafter) of the film-like non-magnetic support member.
As such a magnetic recording medium, a so-called coating type magnetic recording medium in which a magnetic coating material including a magnetic powdery material dispersed in a binder is coated on a film-like non-magnetic support member and then dried to form a magnetic layer is popularly used. In the coating type magnetic recording medium, the protective layer or the back-coat layer is formed in the same manner as that of the magnetic layer. More specifically, the protective layer or the back-coat layer is formed in such a manner that a protective layer material or a back-coat layer material including carbon dispersed in a binder is coated on a non-magnetic member and then dried. At this time, the back-coat layer is generally coated with or includes a lubricant containing a fatty acid as a main component.
In contrast to this, with a demand of high-density magnetic recording, a so-called deposition type magnetic recording medium in which a magnetic layer is formed by depositing a ferromagnetic metal material on a non-magnetic support member has attracted attention.
Such a deposition type magnetic recording medium has the following various advantages. That is, loss of a thickness in a recording/reproducing operation can be considerably decreased because the thickness of a magnetic layer in which saturation magnetization can be increased with respect to the anti-magnetic force of the magnetic layer can be remarkably decreased, and a filling density of a magnetic material can be increased because binder serving a non-magnetic material is not mixed in the magnetic layer.
As such a deposition type magnetic recording medium, the magnetic tape in which a Co.sub.80 Ni.sub.20 alloy is deposited on a film to form a magnetic layer was merchandized in 1989.
In the deposition type magnetic recording medium, as in a coating type magnetic recording medium, a back-coat layer is formed in such a manner that a back-coat layer material obtained by dispersing carbon or the like in a binder is coated on a non-magnetic support member and then dried.
However, in the deposition type magnetic recording medium, since the magnetic layer consists of a ferromagnetic metal material, when the back-coat layer is formed in a conventional manner, the following problem is posed.
That is, in the coating type magnetic recording medium, a lubricant containing a fatty acid as a main component is coated on or included in the back-coat layer to decrease a friction coefficient of the back-coat layer as described above. However, in the deposition type magnetic recording medium having the magnetic layer consisting of a ferromagnetic metal material, when a lubricant containing a fatty acid as a main component is coated on or included in the back-coat layer, the lubricant containing the fatty acid as a main component grows the rust of the ferromagnetic recording medium.
Therefore, in the deposition type magnetic recording medium, the back-coat layer cannot be coated with a lubricant containing a fatty acid as a main component, and cannot include the lubricant. For this reason, the deposition type magnetic recording medium has the following problem to be solved. That is, the friction coefficient of the back-coat layer is decreased without coating or including the lubricant containing a fatty acid as a main component on/in the back-coat layer.
A method of increasing the surface roughness Ra of the back-coat layer to degrading the surface properties may be considered to decrease the friction coefficient of the back-coat layer. When the surface properties of the back-coat layer are degraded, and a tape-like magnetic recording medium is wound, the surface properties of the magnetic layer are also degraded depending on the unevenness of the back-coat layer. In this manner, when the surface properties of the back-coat layer is degraded, the surface properties of the magnetic layer are also degraded due to an influence of the back-coat layer, and degradation of characteristics in a recording/reproducing operation or an increase in dropout is caused. Therefore, the method of degrading the surface properties of the back-coat layer to decrease the friction coefficient of the back-coat layer has the above problem.
In the deposition type magnetic recording medium, the friction coefficient of the back-coat layer must be decreased without coating or including a lubricant containing a fatty acid as a main component on/in the back-coat layer and without degrading the surface properties of the back-coat layer.
In the deposition type magnetic recording medium, the type of a binder used in formation of the back-coat layer is limited. More specifically, for example, a vinyl-chloride-based binder cannot be used because chlorine causes a ferromagnetic metal material to rust. For this reason, in the deposition type magnetic recording medium, since the type of the binder used in formation of a back-coat layer is limited, the appropriate composition of the binder cannot be easily founded.
In the deposition type magnetic recording medium, adhering forces among the magnetic layer, the protective layer, and the back-coat layer considerably change depending on the combination among the material of the magnetic layer, the material of the protective layer, and the material of the back-coat layer. For this reason, these materials must be appropriately determined. If these materials are not appropriately selected, for example, a back-coat layer may be partially peeled, or the back-coat layer may be separated from the support member in a width direction or a traveling direction to loss conductivity. Therefore, the back-coat layer may loss the effects of the back-coat layer.
The present invention has been made in consideration of the above conventional circumstances, and has as its object to mainly improve a protective layer and a back-coat layer of a deposition type magnetic recording medium to prevent the deposition type magnetic recording medium from rust and improving the traveling and still characteristics of the deposition type magnetic recording medium.